For a number of industrial applications, it is useful to determine the surface metrology of samples such as thickness of thin films, their refractive indices and the profile parameters of surface features such as grating on semiconductor wafers. A number of metrology tools are now available for performing optical measurements on semiconductors. Such tools can include spectroscopic reflectometers and spectroscopic ellipsometers. The size of semiconductor devices on silicon wafers has been continually reduced. The shrinking of semiconductor devices has imposed more requirements on the sensitivity of wafer inspection instruments in detecting contaminant particles and pattern defects. One approach to improve the sensitivity of metrology measurements is to employ electromagnetic radiation of shorter wavelengths such as vacuum ultraviolet (VUV) wavelength and ultraviolet-visible wavelength. VUV light is absorbed mainly by oxygen and water molecules generally present in an ambient environment. Therefore, nitrogen or inert gas purging or vacuum is required to extend optical measurement down to vacuum ultraviolet (VUV) range. Examples of such inert gas purging are described, e.g., in US Patent Application Publication 20040150820, which is incorporated herein by reference. Some current broadband spectroscopic ellipsometry (BBSE) systems may use a method of purging Nitrogen gas on/around the measurement spot instead of purging the whole system. However, there is an interaction between the purging N2 gas and physisorbed materials (hydrocarbons, contaminants, etc henceforth referred to as Airborne molecular Contaminants or AMC), which results typically in a change to the effective thickness of the AMC layer on wafer during film thickness measurements in the ultraviolet-visible spectral region, causing the precision of the measurement to be higher.
It is within this context that embodiments of the present invention arise.